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How to Compress a PDF
Compression works by downsampling images and recompressing them — so it only helps PDFs that actually contain large images. Tested 2 November 2026 with Ghostscript 10.02.1: a 4-page photo-heavy A4 report went from 1,851,625 bytes to 342,128 bytes at ebook quality (81.5% smaller) and to 93,687 bytes at screen quality (94.9% smaller). A text-only control PDF went the other way, from 2,667 to 3,933 bytes. Clioos does not offer a compression tool — this guide teaches the free offline method.
At a glance
- Clioos has no PDF compression tool. The method below runs on your own computer with free, open-source Ghostscript — no upload, no account.
- Photo-heavy PDFs shrink a lot (81.5% at ebook quality in the test); text-only PDFs barely shrink and can even grow.
- Ebook quality (150 DPI images) is the safe default; screen quality (72 DPI) is smaller but visibly softer.
- Compression is one-way: keep the original file, because downsampled images can't be restored.
- Password-protected PDFs must be unlocked first.
Steps
- Install Ghostscript: on Linux it is usually already there (
gs --version); on macOSbrew install ghostscript. - Run the ebook preset from the original file:
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.7 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=compressed.pdf input.pdf. - Check the result:
ls -lafor the size andpdfinfo compressed.pdffor the page count. On the test file, 4 pages stayed 4 pages and the size fell from 1,851,625 to 342,128 bytes. - If you need it smaller, re-run from the original with
-dPDFSETTINGS=/screen: the test file fell to 93,687 bytes (94.9% smaller), with visibly softer images. - Open the compressed file and check a photo page before you send it — ebook quality is fine on screen, but small text inside images can soften.
Worked example (tested 2 November 2026)
Test file: a 4-page A4 report with a large photo on every page (1,851,625 bytes). Method: Ghostscript 10.02.1, run from the original file each time — never from an already compressed output.
| Preset | Output size | Smaller by | Pages | Time |
|---|---|---|---|---|
| Original | 1,851,625 bytes | — | 4 | — |
| /ebook (150 DPI images) | 342,128 bytes | 81.5% | 4 | under 1 s |
| /screen (72 DPI images) | 93,687 bytes | 94.9% | 4 | under 1 s |
Control test, same tool and preset: a 3-page text-only PDF went from 2,667 bytes to 3,933 bytes — it grew by 47.5%. That is the honest limit of this method: if your PDF has no large images, compression has nothing to squeeze, and the rewritten file can come out larger. Check the output size before you replace the original.
Which preset to pick
- /screen (72 DPI): smallest files, for email attachments and web uploads where the PDF is only read on screen. Images look soft when zoomed.
- /ebook (150 DPI): the default choice. Readable on screen and acceptable for most printing. This is what the worked example used.
- /printer (300 DPI): for files that will actually be printed. Barely smaller than the original unless the images were oversized to begin with.
- /prepress: keeps colour and resolution for professional printing — not a size-saving preset.
What it can't do
- No Clioos tool for this: Clioos does not offer PDF compression. PDF Probe converts tables to Excel; it does not compress, split or render PDFs. The method above is the free offline route.
- Text-only PDFs barely shrink: the control test grew from 2,667 to 3,933 bytes. If your PDF is mostly text, don't expect a smaller file.
- Quality loss is permanent: downsampled images can't be restored. Always compress a copy and keep the original.
- Scanned pages stay scans: compression doesn't OCR anything. A compressed scan is still a picture, not searchable text.
- Locked PDFs: a password-protected PDF must be unlocked first; Ghostscript can't open it on its own.
- It doesn't fix a broken PDF: if the file is corrupt or won't open, compression won't repair it.
Free alternatives, honestly compared
- qpdf (free, offline, documented):
qpdf --object-streams=generate input.pdf output.pdfcleans up structure and can shrink some files, but it doesn't downsample images — for photo-heavy PDFs, Ghostscript wins by a wide margin. - Browser tools (iLovePDF, Smallpdf, documented): upload, compress, download. No install, but your file leaves your machine and free tiers have a daily limit — fine for one-offs, not for sensitive documents.
- macOS Preview (documented): File → Export, choose Quartz Filter → Reduce File Size. One click, but you can't pick the DPI — and the result is often softer than Ghostscript's ebook preset.
Frequently asked questions
Is it free to compress a PDF?
Yes, with Ghostscript — free and open source, no per-file charge. Browser tools are free up to a daily limit.
How much smaller will my PDF get?
It depends on the images inside. A photo-heavy PDF shrank 81.5% in our test; a text-only PDF grew. Run the command and check the output size — don't assume.
Will compression hurt quality?
At ebook quality, text stays sharp and photos look fine on screen. At screen quality, images soften visibly when zoomed. Keep the original either way.
Can I compress a PDF on my phone?
Ghostscript needs a computer. On a phone, browser tools work, but they upload your file to their server.
What if the PDF is password-protected?
Unlock it first with the password. Compression tools can't open a password-protected file on their own.
Related guides: How to split a PDF · How to merge PDF files · How to convert PDF to PNG · All guides · Tool: PDF Probe